Apparatus and method for checking mechanical parts

ABSTRACT

The geometrical features of a part (6) having center seats are electronically checked by processing signals of gauging heads (14-17) that relate to linear dimensions of the part together with signals of other gauging heads (30, 31) adapted to define a reference geometrical axis. In order to obtain checkings of high accuracy, the reference geometrical axis is defined by means of two gauging heads (30, 31), each of which has a feeler (67, 72) movable along the three directions that is adapted to cooperate with a respective center seat.

TECHNICAL FIELD

The invention relates to an apparatus for checking geometrical featuresof mechanical parts with symmetry of rotation and having two centreseats, with first supporting means for supporting the part to bechecked, by cooperating with it at positions different from said seats;second supporting means; gauging means fixed to the second supportingmeans and adapted to cooperate with the part; and processing meansconnected with the gauging means for determining said geometricalfeatures.

The invention also relates to a method, for checking the concentricityof cross-sections of a part having symmetry of rotation and featuring atits ends centre seats, according to which centres of said cross-sectionsare automatically determined by electronically processing the signals ofgauging heads for measuring linear dimensions, a reference geometricalaxis is electronically defined and the distances of said centres fromthe reference geometrical axis are determined.

BACKGROUND ART

It is known to check, statically or dynamically (i.e. with the partbeing stationary or in rotation), the concentricity of differentcross-sections of mechanical parts with symmetry of rotation, inparticular shafts, with reference to a geometrical axis defined by atotally mechanical manner.

The geometrical axis is normally defined by two precision centres (withrelevant support devices) that cooperate with the corresponding seatsobtained at the ends of the shaft for permitting support and rotation ofthe latter during the subsequent machining phases (turning, grinding,etc.). One of the centres can be motorized, for performing dynamicalcheckings.

In several cases, from a theoretical point of view it is more correct totake as a reference the geometrical axis defined by the centre seats,but it is difficult and expensive to make high precision centres andrelevant support devices, for preventing displacements of saidgeometrical axis with respect to the gauging devices or heads thatperform dimensional measurements in correspondence with the differentcross-sections of the shaft.

Therefore, in other cases the checking is performed by taking as areference a geometrical axis defined as the axis passing through thecentres of two cross-sections of the part. For example, for acrankshaft, the reference axis can be determined by processing thesignals of two gauging heads each of which has two feelers in contactwith a respective journal for determining two radial dimensions of thejournal along the same diametral direction.

The signals of these gauging heads are combined with those of furthergauging heads that check other radial dimensions of the part. The partcan be supported by Vee supporting devices or also by low precisioncentres, since possible spatial displacements of the axis ofinstantaneous rotation of the part are electronically compensated.

However, as already mentioned, sometimes the measurement problemsrequire to take as a reference the axis defined by the seats for thesupporting centres, in particular for inter-operational checkings of themachining process.

DISCLOSURE OF INVENTION

Object of the invention is to permit checking of geometrical features,such as concentricity, of parts with symmetry of rotation, e.g. shafts,with reference to a geometrical axis defined by the relevant centreseats, while guaranteeing high repeatability and accuracy, without anyneed of supporting the part by high precision centres and relevantsupports.

The invention as characterized in the claims solves the problem ofdetermining in an electronic way the position of the geometrical axis ofa part with symmetry of rotation as defined by the relevant centreseats, while the part is supported by supporting devices different fromthe centres.

Since the position of the axis defined by the centre seats is readilyand continuously determined, possible displacements of the same axis canbe compensated. Therefore the part can be supported and rotated by lowprecision devices. It is possible to detect displacements of said axisalong one or two coordinates. Moreover, it is quite easy to complete theapparatus and the method according to the invention in such a way as tocheck the distance between the centre seats.

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described in detail with reference to a preferredembodiment, illustrated in the annexed drawings, given as a non limitingexample only, in which:

FIG. 1 is a schematic front view of a gauging bench for checkingdimensions and geometrical features of shafts;

FIG. 2 is a schematic cross-section of the bench of FIG. 1, along pathII--II in FIG. 1;

FIG. 3 is a plan view, with enlarged scale, of a detail of the bench ofFIGS. 1 and 2, comprising a gauging cell with a feeler adapted tocooperate with a centre seat of a shaft; and

FIG. 4 is a cross-section of the detail of FIG. 3 according to the planealong path IV--IV in FIG. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

The gauging bench of FIG. 1 comprises first supporting means including abase 1, that supports, through vertical supports 2 and 3, two Veedevices, 4 and 5, for supporting and providing a reference to the partsto be checked. In the specific case, the parts are constituted by shafts6 comprising a plurality of sections having diameters of differentvalues.

Shaft 6 is inserted from above onto Vee devices 4, 5. The loading ofpart 6 can be manual or automatic and is rendered easier by guidingelements 7, 8 that are also fixed to base 1.

Two arms 9, 10, rotatable about horizontal axes or pins 11 arrangedunder shaft 6 (when shaft 6 is in the measurement position), see FIG. 2have free ends coupled to second supporting means including a supportingand guiding frame 12.

Frame 12 supports measuring means comprising gauging heads 14, 15, 16and 17, each having two feelers adapted to enter into contact withdiametrically opposite points of shaft 6.

By way of example, as shown in FIG. 2, gauging head 14 comprises twoarms 20, 21 movable about integral fulcrums 22, 23 and carrying at theirends feelers 24, 25. To each movable arm 20, 21 and to an intermediatesupport 26 there are coupled elements of two position transducers 27,28, that provide measurement signals responsive to the deviations, fromthe nominal value, of the radiuses of the section of shaft 6corresponding to feelers 24, 25.

Arms 9, 10 are coupled to other arms carrying control knobs, but this isnot shown, for simplicity's sake. The rest and operating positions ofarms 9, 10, with the corresponding gauging heads 14-17, are defined bymechanical stops, not shown. The rest position is partially shown inFIG. 2 by short dashes.

The measuring means carried by frame 12 also comprise two gauging cells30, 31, the second of which is more clearly shown in FIGS. 3 and 4.

Gauging cell 31, similarly to cell 30, is a three axis gauging cell,or--more properly--a cell featuring three degrees of freedom.

A first portion 32 of cell 31 comprises a base 33 fixed to frame 12 byscrews 34, 35. Portion 32 also includes two arms 37, 38 integrallyconnected to base 33. Arm 37 is fixed with respect to base 33, while arm38 can perform limited rotational displacements about a geometricalaxis--perpendicular to the plane of FIG. 3 and parallel to the plane ofFIG. 4--defined by a section having reduced thickness, or integralfulcrum 39.

The mutual displacements of arms 37, 38 are limited by limiting devices40, 41.

The mutual rest position of arms 37, 38 is determined, among otherthings, by a spring 42. The deviations of arm 38 from this rest positionare measured by a position transducer 43 having two parts respectivelyconnected to arms 37, 38.

A second portion 44 of cell 31 is connected to the free end of arm 38 inan integral way (or through screw devices and/or welding). Portion 44comprises a base 46 and an arm 47 fixed with respect to arm 38 and anarm 48 that can perform limited rotational displacements about ageometrical axis, arranged perpendicularly to the plane of FIG. 3 and,with reference to FIG. 4, lying horizontally and in the plane of thesame FIG. 4. This geometrical axis is defined by a section havingreduced thickness, or integral fulcrum 50.

Coupled to arms 47, 48 are two limiting devices 51, 52, a spring 53coupled to limiting device 52 and a position transducer 54.

A third portion 56 of cell 31 comprises an arm 57 integrally and rigidlysecured to arm 48 and an arm 58 integrally joined to arm 48 through asection having reduced thickness, or integral fulcrum 60. Integralfulcrum 60 defines a rotation geometrical axis arranged, with referenceto FIG. 3, horizontally and substantially in the plane of the same FIG.3 and, with reference to FIG. 4, perpendicularly to the plane of FIG. 4.

Coupled to arms 57, 58 are two limiting devices 62, 63, a spring 64 anda position transducer 65.

A feeler 67 having a hemisphere shape is fixed at the end of movable arm58.

The diameter of the hemisphere defining feeler 67 has a pre-set valuesuch as to enable the proper cooperation with the end 70, having theshape of a truncated cone, of a centre seat ("centre") obtained at anend of shaft 6.

The operation of the bench of FIGS. 1-4 is as follows.

With arms 9, 10, frame 12, gauging heads 14-17 and cells 30, 31 in restposition, shaft 6 is horizontally loaded onto supporting devices 4 and5.

Arms 9, 10 and the other elements associated with them are displaced tothe operating position by controlling the previously mentioned knobs.Consequently, feeler 67 and a corresponding feeler 72 (visible inFIG. 1) of cell 30 enter into seat 70 and into another centre seat("centre") obtained at the other end of shaft 6. The insertion offeelers 67, 72 is guided by two reference devices 75, 76, visible inFIG. 1, fixed to supporting devices 77, 78 coupled to base 1. Referencedevices 75, 76 also define the rest position of feelers 67, 72.

Of course, also the feelers of gauging heads 14-17 reach the measuringposition, into contact with diametrically opposite points ofcorresponding cross-sections of shaft 6.

Then shaft 6 is driven into rotation (through at least 360°) manually(or possibly by a motorized device) on supporting devices 4, 5. Duringthis rotation transducers 43 and 65 and the corresponding transducers ofgauging cell 30 detect possible displacements--along two perpendiculargeometrical axes lying in a plane perpendicular to the geometrical axisof shaft 6--of the geometrical axis of shaft 6 defined by thecooperation among feelers 67, 72 and the corresponding centre seats ofshaft 6.

Transducer 54 and the corresponding transducer of gauging cell 30 permitchecking deviations of the length of shaft 6, in correspondence with thecentre seats, from the nominal value, through per se known processing ofthe relevant signals in processing means comprising a processing anddisplay unit 80, shown in FIG. 1.

This unit 80 also receives, through conductors housed within a cable 81,the signals of the transducers of gauging heads 14-17 and the signals ofthe other transducers of gauging cells 30 and 31. By processing in aknown way these signals it is possible to obtain the deviations of thediameters of the various sections of shaft 6 from the correspondingnominal values and the components of the possible concentricity errorsof the centres of the same sections with respect to the geometrical axisdefined by feelers 67 and 72.

Gauging cells 30 and 31 guarantee high accuracy, because thedisplacements of the feelers 67 and 72--that substantially take placealong three perpendicular directions and therefore, by composition, inthe space--are obtained through integral fulcrums, without plays andfriction. Only the contact of feelers 67 and 72 with the correspondingcentre seats involves friction, the influence of which can however berendered negligible by a suitable design and the precision machining ofgauging cells 30 and 31.

Of course, gauging cells 30 and 31 can be provided with a singletransducer whenever it is sufficient to measure one component of thedisplacements of the axis defined by the centre seats, or with twotransducers if it is needed to measure the two components of thesedisplacements, or one component and the length of shaft 6.

I claim:
 1. Apparatus for checking geometrical features of a mechanicalpart with symmetry of rotation and having two end centers, with firstsupporting means (1-5) for supporting the part to be checked, bycooperating with it at positions different from said end centers; secondsupporting means (12); gauging means (14-17, 30, 31) fixed to the secondsupporting means and adapted to cooperate with the part (6); andprocessing means (80) connected with the gauging means (14-17, 30, 31)for determining said geometrical features, wherein said gauging meanscomprise two gauging heads (30, 31), each of which has a base fixed tothe second supporting means and a feeler (67, 72) movable substantiallyin three orthogonal directions with respect to the base and adapted tocooperate with a corresponding one of said end centers (70), theprocessing means (80) being connected to said gauging heads (30, 31) fordetermining the position of the geometrical axis of the part defined bysaid end centers.
 2. An apparatus according to claim 1, wherein thefirst supporting means (1-5) comprise Vee reference devices (4, 5)adapted to support the part (6) and permit its rotation.
 3. An apparatusaccording to claim 1, wherein said gauging means (14-17, 30, 31)comprise further gauging heads (14-17) adapted to provide measurementsignals for combined checkings and the second supporting means comprisea common support (12) for said two gauging heads and the further gaugingheads.
 4. An apparatus according to claim 3, wherein said furthergauging heads (14-17) provide signals responsive to at least two radialdimensions of the part (6) in correspondence with a diametricaldirection in a cross-section of the part, and said two gauging heads(30, 31) provide signals responsive to displacements of said axis of thepart along said diametrical direction, the processing means (80) beingadapted to process the signals of said further gauging heads (14-17) andof the two gauging heads (30, 31), for measuring concentricity errors ofsaid cross-section with respect to said axis of the part.
 5. Anapparatus according to claim 1, wherein each of said two gauging heads(30, 31) comprises position transducer means (43, 54, 65) for detectingdisplacements of the corresponding feeler (67) along at least one ofsaid three directions.
 6. An apparatus according to claim 1, whereineach of the two gauging heads (30, 31) comprises two positiontransducers (43, 54, 65) for detecting displacements of the feeler (67)substantially along the direction of the geometrical axis of the part(6) and along a direction perpendicular thereto.
 7. An apparatusaccording to claim 6, wherein the processing means (80) are adapted tocheck the length of the part (6) in correspondence with the feelers (67,72) of said two gauging heads (30, 31).
 8. An apparatus according toclaim 1, for checking the part (6) in which said end centers have aconical surface (70), wherein said feelers (62, 72) have a substantiallyspheric shape for guaranteeing centering with respect to the conicalsurfaces of the end centers.
 9. An apparatus according to claim 1,wherein the processing means (80) are adapted to check said geometricalfeatures with the part rotating on the first supporting means (1-5). 10.An apparatus for checking geometrical features of a mechanical part withsymmetry of rotation and having two end centers, with first supportingmeans (1-5) for supporting the part to be checked by cooperating with itat positions different from said end centers; second supporting means(12); gauging means (14-17; 30, 31) fixed to the second supporting meansand adapted to cooperate with the part (6); and processing means (80)connected with the gauging means (14-17; 30, 31) for determining saidgeometrical features, wherein each of said gauging means comprises twogauging heads (30, 31), each of which has a feeler (67, 72) adapted tocooperate with a corresponding one of said end centers (70), the feelerbeing movable substantially in three directions, a movable arm carryingthe feeler, the movable arm being coupled to the base through threeintegral fulcrums (39, 50, 60) for permitting, respectively, thedisplacements of the feeler along said three directions, the processingmeans (80) being connected to said gauging heads (30, 31) fordetermining the positions of the geometrical axis of the part defined bysaid end centers.
 11. A method for checking the concentricity ofcross-sections of a part, the part (6) having a shape with symmetry ofrotation and being provided with end centers, including the steps of:arranging the part in a determined measurement position by supporting itat locations different from said end centers; measuring lineardimensions of the part at said cross-sections by first gauging means(14-17) providing electronic measurement signals; electronicallyprocessing said measurement signals for detecting the centers of saidcross-sections; electronically determining a geometrical reference axisby directly detecting the positions of the end centers by two gaugingheads (30, 31), each gauging head having a base and a feeler directlycontacting one of the end centers, the feeler being movable with respectto the base along three substantially orthogonal measurement directions;and determining the distances of the centers of said cross-sections fromsaid geometrical axis.